The use of scraper devices at the bottom of digesters and impregnation vessels in the continuous cooking of chemical cellulose pulp has been long known. The aim of these scraper devices is to ensure a continuous output of the cellulose pulp or chips from the vessel. The scraper device consists of a number of scraper arms that are arranged on the shaft that is arranged to be vertical during production. The motion of the arms in the suspension of pulp or chips counteracts the formation of blockages, the formation of channels, and other undesired effects.
The above-mentioned shaft for the operation of the said scraper arms has been used since early times for the addition of fluid at the lower part of the digester or impregnation vessel. The addition of fluid occurs in this case by making the shaft hollow and leading fluid in through this way. The primary purpose of adding fluid has been to wash the pulp. This addition of fluid through the shaft has more recently been used for the dilution of the pulp with the aim of ensuring output from the vessel. U.S. Pat. No. 5,736,005 reveals a variant of such a hollow shaft in which fluid is added to a continuous digester with the aim of ensuring output from the digester.
An alternative to the above-described addition of fluid with the aim of diluting and ensuring output of the pulp or chips from the digester or the impregnation vessel is to add the fluid at the lower part of the vessel through a fluid supply device through the vessel. It is preferable that this addition takes place in the vicinity of the scraper device. SE 180 289 reveals an embodiment in which the fluid supply device adds fluid close to the bottom of a container with the aim of preventing the formation of blockages of cellulose fibres.
Addition of fluid by the methods that have been described above, however, involves a number of disadvantages, particularly when the addition is made to an impregnation vessel.
In the cases in which the fluid is added to an impregnation vessel, the extra addition of fluid must be dealt with by the top separator in subsequent digesters, which involves a considerable extra expense at the top separator.
Furthermore, the added fluid involves large volumes of fluid that the system must deal with, and this in turn involves expensive investment and high operating costs of pumps and high-pressure taps, or both.
The same problem arises, naturally, also in those cases in which no fluid has been added at the bottom of the impregnation vessel due to the fluid/wood ratio of the chips suspension being so high that it is not necessary to add fluid in order to ensure output from the impregnation vessel.